This invention relates to ultrasound scanning, more particularly to digital scan converters and interpolation methods and their implementation.
When displaying echo signals representative of the amplitude of ultrasound energy returning from spaced apart paths distributed in an image space on a conventional raster scan display, there are usually many more display pixels (picture elements) than there are acquired image samples from the image space. This in part is due to fundamental constraints on the velocity of sound in the image object, such as human tissue in medical applications, and to the space and time resolutions of the transmitted ultrasound pulses. This results in a need to reconstruct the image space by interpolating between echo samples, that is, by assigning gray scale values to the interpolated pixels.
The above problem is further aggravated when employing sector scanning where the field of view spreads out as the depth of penetration increases. More acquired image data are available for use in interpolation and display near the pole of the sector while much less is available at the bottom of the sector. One dimensional along the raster (ATR) interpolation is one common scheme used in scan converters. The advantage of this method is low-cost; however, usually the image resolution is deteriorated; artifacts are generated; and the image has a very poor quality. In published UK patent application No. GB 2089 537 A a scan converter apparatus is disclosed in which at least four data samples along radial lines are used to interpolate data to be displayed along display points lying along orthogonal rows and columns. The use of four data samples helps alleviate the above mentioned problems of one dimensional along the raster (ATR) interpolation approaches.
However, not only should digital scan converters provide better resolution through more accurate interpolation but they should do so in a relatively inexpensive and simplified manner, and they should provide additional needed features not available or not easily implemented with non-digital converters. For example, digital scan converters should operate for either linear or sector scanning; provide for operation of various sizes of sector image spaces; provide for scaling between the image and display spaces so that the full display space is available for displaying only a portion of the image space or the full image space is displayed in only a portion of the display space; and provide for correction of other deterministic errors beside interpolation errors, such as, correction of certain errors associated with wobbler scanheads, etc.